1. Field of the Invention:
This invention relates to heavy duty pneumatic radial tires and is to diminish railway wears being apt to occur in such tires when continuously travelling at a high speed over a long distance.
2. Description of the Prior Art:
In general, radial tires of this type provided with metal cords as a belt reinforcement have excellent resistance to wear, puncture and the like as compared with conventional bias tires because they have a highly stiff belt arranged between the tread rubber and the carcass ply. On the other hand, the radial tires are somewhat defective in the comfort degree owing to the rigid reinforcing effect with such belt. Accordingly, these radial tires have been developed for use on good road aside from bad road. Recently, the demand for such tires in considerably increased in association with the remarkable improvement of road circumstances such as the development of networks of highways and the like.
In such applications, zigzag-type ribs extending circumferentially of tire are provided in the tread of tire considering traction and braking performances, general wear resistance, resistance to heat build-up and the like. Generally, the tire having such tread pattern is called as a rib-type tire.
In the rib-type tire, ribs are usually continuous toward the circumferential direction of tire and may be discontinuous toward the circumferential direction due to the presence of traverse grooves arranged along the widthwise direction of tire. In any case, when a vehicle provided with such tires goes continuously straight at a high speed over a long distance, there are caused extraordinarily irregular wears (hereinafter referred to as eccentric wear) which have never been observed in the conventional tires.
Namely, as shown in FIG. 1, the eccentric wear is locally caused in a shadowed region A near a top of a convex part 3 of a circumferential rib 2 formed in a tread of a tire T, said convex part being projected in a widthwise direction of the tire T toward a main groove 1 extending zigzag along a circumferential direction of the tread, and then gradually increases to form a region A having a stepwise height h and a width w in section as shown in FIG. 2. The region A of the eccentric wear gradually grows with the increase of the travelling distance and finally communicates with adjoining regions A. As a result, these regions are continuously joined with each other in the circumferential direction of tire. Moreover, the stepwise height h and the width w are gradually enlarged with the increase of the travelling distance.
The above eccentric wear is generally called as a railway wear, which produces not only the recess on the main groove 1 to render the appearance of the tire T awkward, but also adversely affects tire performances, particularly traction and braking performances depending upon the function of an edge line of the rib 2 partitioned by the main groove 1 and further considerably deteriorates the life of tire.
The eccentric wear begins to occur only in the vicinity of the top at the convex part 3 of the zigzag-type circumferential rib 2 and does not start from a concave part 4 of the circumferential rib 2 in opposition to the convex part 3 along the widthwise direction of the tire T. However, the concave part 4 is also subjected to railway wear in due time with the evolution of the eccentric wear.